1. Field of the Invention
The present invention relates to a thermal printing method and thermal printer for thermal recording of an image to a thermosensitive recording material. More particularly, the present invention relates to a thermal printing method and thermal printer capable of high-speed printing while a recording surface of the recording material is prevented from being damaged.
2. Description Related to the Prior Art
There are widely used thermal printers including a direct thermal printer in which thermosensitive recording material is heated for directly printing an image. The color thermosensitive recording material or recording sheet adapted to the direct printing is constituted by a support and thermosensitive coloring layers of magenta, yellow and cyan which are formed on the support. Selectively to develop colors of the coloring layers, the coloring layers are different in thermal sensitivity. A superficial one of the coloring layers, located with a recording surface of the recording sheet, has the highest thermal sensitivity. The coloring layers have lower thermal sensitivity according to the closeness to the support. Amounts of heat energy applied to the recording material are controlled, to color the coloring layers differently in coloring density.
To print a full-color image, the recording material is moved while a thermal head is pressed against the same and generates heat. At first a yellow image is recorded one line after another by coloring the yellow coloring layer. Immediately after the yellow recording, near ultraviolet rays peaking at 420 nm are applied to the recording material, to fix the yellow coloring layer. Then a magenta image is recorded one line after another by coloring the magenta coloring layer. Immediately after the magenta recording, ultraviolet rays peaking at 365 nm are applied to the recording material, to fix the magenta coloring layer. Finally a cyan image is recorded by coloring the cyan coloring layer.
It is conceived to heighten printing speed in a thermal printer. For example it is conceived to convey the recording material at a high conveying speed of 16 mm/sec during the printing. To obtain the coloring density obtainable according to conventional printers, the thermal head is required to apply the heat energy in a shortened period to the recording material. In the case of coloring the cyan coloring layer the closest to the support nearly at the maximum density, the heat energy applied by the thermal head to the recording material is so high that the thermal head has high temperature. There occurs thermal roughening on the recording surface of the recording material. The thermal roughening consists of small bubbles created in a protective layer of the recording material covering the yellow coloring layer due to the contact with the thermal head. The thermal roughening lowers gloss of the recorded image on the recording surface to degrade the image quality, and lowers the coloring density of the printed image because the bubbles whiten the protective layer.
Also, water vapor is likely to emerge from the cyan coloring layer due to the high temperature of the thermal head in contact with the recording material. The vapor is accumulated between the support and the cyan coloring layer to create "blisters" which are comparatively greater than the surface bubbles. The blisters damage the appearance and quality of the recording material in addition to the decrease of the coloring density of the printed image. There is no prior technique capable of reliably minimizing the decreases in the gloss and/or the coloring density.